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Weeping head at low spot in zone, not the diaphram?

I have a very slowly weeping head on the middle zone(#3) in a 6 zone system. All 6 valves are located above ground in the same location.
1) I initially replaced the diaphram on the zone #3 : Still wept.
2) replaced the solenoid on #3 valve : Still wept
3) Replaced the entire valve and solenoid assy. with a new one : Still wept.
4) Removed brand new diaphram, inspected and lubricated : Still wept.
5) Cut the 3/4" pvc pipe exit from zone#3 just downstream of valve #3. Exercise valve on/off several times. When in off postion : no weaping.
6) Connected valve assy removed previously on house bib and it did not weep when off.
7) Installed a pressure gauge to confirm upstream pressure is 60 -70 lbs on manifold. Exercised all zones one at a time while watching head in zone #3 that had wept previously : Initially saw a little weeping when zone 2 was turned off.
Was later unable to duplicate this.

After several days now with the zone #3 piping left open at valve outlet, no weeping. Other zones have been running every day.

I am now ready to reconnect the zone #3 valve outlet once again. Why do I expect it to weep once again ????

At wits end. What am I missing? I was thinking that somehow the zones were unintentionally partially connected together downstream of the valves. Testing does not support this. Any other tests recommended before I reconnect and hope for the best?

There is a master shut off (manual valve)

Just wanted to clarify that there is a master shut off (manual valve).
That was one troubleshooting step I forgot to mention.

When the system was still plumbed intact, and weeping, shutting off the master manual valve did stop the weeping at zone #3. So I know the water source is not external to the system.
When the master was turned back on the weeping restarted.

It could be the controller causing the weeping. I've seen where the controller sends out a weak signal to the valve 24 hours a day causing the valve to weep.
The problem went away with the installation of a new controller.

It could be the controller causing the weeping. I've seen where the controller sends out a weak signal to the valve 24 hours a day causing the valve to weep.
The problem went away with the installation of a new controller.

It was an Irritrol MC-18A. The old controllers in the yellow metal cabinet.
That type controller seems to have this problem.
I've seen it a few times but only the one time did it cause weeping.
I'd already changed the valve. The new timer fixed it.

It is not the controller. Had disconnected the wires from the solenoid and it still weeped. Also measured 0 volts out of zone three while weeping.

Sure I could have installed a quality automatic master valve and Installed a new controller to control it. Current controller is only 6 zone and all are used. It would also have required me to replumb the entire manifold in order to insert the new valve.
I was hoping for an experienced person to respond who might have encountered such an issue and had a suggestion for the correct fix.

The reason I have spent the time is that I am an engineer and always desire to know why something is failing rather than to simply mask the problem.
I do understand however people who do this for a living have to make a living and often times the quickest fix is the best.

After temporarily replumbing the zone #3 valve to its heads, and after running a complete sequence, the zone #3 head is once again weeping.

Here is a theory on why this is happening if anyone might care to offer a previous experience that might support the theory.

Theory: Zone #2 piping has somehow developed a slight and unintentional connection to the zone #3 piping downstream from the valves. And Zone #2 must be weeping but I did not see that because the lowest point in the system was zone #3.

I know that Zone#2 and Zone #3 pipes run parallel to each other for over 30 ft or so along side of the house. I know this from when I replaced a zone #2 head on the side of the house a few years back. When I dug up the head there were (3) 3/4" pvc pipes in close proximity to each other near that head. There was also a large tree root that had grown around the piping and that head.

Also to support this theory it seems that zone #2 and zone #3 must be run for a few minutes to establish the flow. Maybe possible that zone #2 must be full of water and zone #3 running creates a vacuum that begins the suck from zone #2 into #3.

I know this may sound like a stretch but it is my best guess at this point.

My next step is to focus on zone #2 diaphragm and valve assy. to see if it might be weeping.